Ever since the financial crash of 2008, we've been having an anxious national debate about the growing income gap. What does it mean for American society when most workers' wages are flat and almost all economic gains flow to the top 1% — or to the top one-tenth of 1%?

But it's a discussion that too often turns into a sterile squabble over economic theology between liberal populists, who want to tax the rich, and supply-siders, who say that cutting taxes will add jobs, reward entrepreneurship and raise all boats.

So maybe it would be helpful to set the problem of the income gap aside for a while and focus instead on a problem everyone can agree on: the opportunity gap.

The American dream is the idea that anyone can get ahead in life with talent and hard work. But that ideal of wide-open opportunity has been dented quite a bit in the economic stagnation of the last few years.

And that's a problem that Republicans as well as Democrats recognize.

"What I believe unites the people of this nation … is the simple, profound belief in opportunity for all — the notion that if you work hard and take responsibility, you can get ahead," President Obama said in his State of the Union address this year. "Let's face it: that belief has suffered some serious blows."

The income gap gets the headlines, but it's the opportunity gap that should concern us all.— Sen. Rob Portman (R-Ohio)

"The income gap gets the headlines," said Sen. Rob Portman (R-Ohio) in a speech last week. "But it's the opportunity gap that should concern us all."

In recent months, Rep. Paul D. Ryan (R.-Wis.), Sen. Marco Rubio (R-Fla.) and other potential GOP presidential candidates have made a point of calling for a new conservative agenda to help the poor and the struggling middle class. Their ideas all build around the emollient idea of increasing opportunity for everyone.

"If we focus on the question of opportunity, the two political parties may actually be able to have a respectful, fact-based discussion, even though they obviously won't agree on every solution," said William A. Galston, a former Clinton aide now at the Brookings Institution.

If only it were that easy.

But here's a starting point for that discussion: education.

Education has long been the traditional route to opportunity for American families of modest means. But a growing educational achievement gap between low-income and affluent kids is making that path both harder and less accessible.

And the gap is getting wider, mostly because wealthy kids' test scores have been improving dramatically while middle-class kids' have improved only slightly over time. "The top has pulled away from the middle," says Sean Reardon of Stanford's Graduate School of Education.

Strikingly, much of that income differential in test scores shows up among kids who are tested in the first months of kindergarten, before they've spent significant time in school. "It's preschool," Reardon said, along with "the out-of-school environment, that creates the gap." Affluent kids are far more likely to get a good preschool education and have parents who read to them and nurseries full of educational toys.

And the gap only widens from there. Other scholars found in 2006, that parents in the top one-fifth of households spent about $7,500 more each year on their children — on child care, tutors, after-school programs and athletics — than households in the bottom fifth. Much of that spending, of course, was designed to improve kids' chances of getting into a good college, the biggest tollbooth on the opportunity trail.

Canadian writer Chrystia Freeland calls this "the educational arms race."

"We're all going to do everything we can for our kids, of course; there's no way to stop that," she told me. "But it creates a huge difference in where people are at the starting line."

One obvious way to address the problem would be with free, high-quality preschool for all children. Thirty states have already established universal preschool programs, including conservative Oklahoma and Georgia.

So far, Republicans in Congress have resisted Obama's proposal for a federal program that would help fund pre-K programs across the entire country. But if they start seeing results from states that aren't controlled by Democrats, perhaps they can be brought around.

Another important element for leveling the educational playing fields is community colleges, which have historically been a route both to four-year degrees and to better jobs for low-income students.

But community colleges are struggling in both blue and red states. During the recession, states made deep cuts in their funding. The Center on Budget and Policy Priorities reported this month that state spending on both two-year and four-year public colleges has declined 23% per student since 2007.

Providing universal preschool and improving community colleges won't close the opportunity gap. But with both parties agreeing that the gap needs attention, they might provide a starting point for creative politicians to find some common ground — and even to seize an opportunity.